The present invention relates to a method for die bonding a semiconductor device onto a substrate. The present invention has particular applicability in assembling semiconductor devices into very thin packages having non-metallic lead frames.
In conventional semiconductor packaging techniques, a bare semiconductor device, called a xe2x80x9cdiexe2x80x9d, is attached to a substrate, such as a metal lead frame, using an adhesive die attach material such as epoxy in a process known as die bonding. The epoxy is typically applied to the lead frame as a wet paste prior to placing the die on the lead frame. Thus, a layer of epoxy separates the die and the substrate. After die bonding, wires are attached to electrically connect the semiconductor device to the lead frame, then the semiconductor device is encapsulated in a plastic material, as by molding.
Newer packaging technologies with reduced package thickness employ flexible non-metallic film-type lead frames to support the die prior to encapsulation, typically comprising organic materials such as polyimide (i.e. plastic) with embedded conductors. It has been determined that the reliability of flexible lead frame packages is improved when the die is separated from the lead frame a predetermined amount, such as 3-5 mils, as by the die attach material (epoxy). Thus, it is advantageous to apply the epoxy paste to a thickness of about 3-5 mils during the die bonding process. The thickness of the epoxy paste is known as the xe2x80x9cbondline thicknessxe2x80x9d. Due to the semi-liquid nature of the epoxy paste, the die typically sinks when it is placed on top of the paste, resulting in inadequate bondline thickness; e.g., only about 1 mil, which adversely affects reliability. Moreover, it is not possible to achieve a consistent bondline thickness from one device to another.
To achieve adequate and consistent bondline thickness, spacers can be incorporated into the die attach paste. However, this requires a specially formulated paste, which has different physical characteristics than conventional paste, thus requiring re-engineering of the die bonding process and raising the cost of the die bonding process. Furthermore, the spacers remain in the finished device, and can adversely affect reliability. Alternatively, thin rigid lead frames may be used instead of flexible lead frames. However, as packages are further reduced in size, such a solution will not continue to be viable, due to technical limitations arising in rigid substrates relating to the formation of fine-geometry conductors. Therefore, flexible film lead frames will need to be employed.
There exists a need for a cost-effective methodology for consistently bonding semiconductor dies to substrates such that each die has a predetermined standoff from the substrate to which it is bonded, without adversely affecting the reliability of the finished devices.
An advantage of the present invention is a method of die bonding a semiconductor device to a substrate which provides adequate and consistent bondline thickness, assuring that the die is spaced from the substrate a predetermined amount without adversely affecting the reliability of the finished device and without significantly increasing manufacturing costs.
Additional advantages and other features of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The advantages of the invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a method of assembling a semiconductor device to a bonding area of a substrate, the method comprising positioning a spacer of a predetermined thickness on a first portion of the bonding area; applying an a die attach material on a second portion of the bonding area spaced apart from the first portion; placing the semiconductor device over the bonding area and in contact with the die attach material and the spacer; heating to cure the die attach material; and removing the spacer, thereby forming a gap between the semiconductor device and the substrate about equal to the predetermined thickness at the first portion of the bonding area.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.